1. Field of the Invention
The present invention relates to a method and an apparatus for driving a plasma display panel, and more particularly to a method and an apparatus for driving a plasma display panel for realizing a high contrast information display terminal, flat television or the like by using a plasma display panel with high definition and large display capacity.
2. Description of the Related Art
Generally, a plasma display panel (referred to as PDP hereinafter) has a thin structure, is free from flickering and has a high display contrast ratio. Moreover, it has many features such as, that it can be made into a relatively large screen, a high response speed, is self-luminous and can be made multi-color luminous by the use of phosphors. Because of this, it has been used widely in recent years in the field of computer related display devices, in the field of color image display devices or the like.
According to the mode of operation, PDPs can be classified into two groups, namely, those of AC discharge type in which the electrodes are covered with a dielectric and is operated under the condition of indirect AC discharge, and those of DC discharge type in which the electrodes are exposed to the discharge space and is operated under the condition of DC discharge. The AC discharge type is further classified according to the driving method into memory operation type which utilizes the memory function of discharge cells and refresh operation type which does not utilize the function. The luminance of a PDP is proportional to the frequency of discharges, that is, the number of repetitions of the pulse voltage. In the refresh operation, type PDP, the luminance-falls off with the increase in the display capacity so that it is used mainly as a PDP of small display capacity.
FIG. 1 is a sectional view showing the configuration of one display cell of a PDP of the AC discharge memory operation type. The display cell is provided with a back face and a front face insulating substrates 1 and 2 made of glass, trace electrodes 5 and 6 which are arranged so as to overlap with a transparent scanning electrode 3 and a transparent sustaining electrode 4 formed on the insulating substrate 2. A data electrode 7 formed on the insulating substrate 1 so as to intersect perpendicularly the scanning electrode 3 and the sustaining electrode 4, a discharge gas space 8 formed between the insulating substrates 1 and 2 filled with a discharge gas such as He, Ne, or Xe or their mixture, barriers 9 for securing the discharge gas space 8 as well as for sectioning a display cell, a phosphor 11 for converting ultraviolet rays generated by the discharge gas into visible rays 10, a dielectric film 12 covering the scanning electrode 3 and the sustaining electrode 4, a protective film 13 made of magnesium oxide or the like for protecting the dielectric film 12 from the discharge, and a dielectric film 14 covering the data electrode 7.
Next, referring to FIG. 1, the discharge operation of a selected display cell will be described. When discharge is initiated by applying a pulse voltage exceeding a discharge threshold between the scanning electrode 3 and the data electrode 7, positive and negative charges are attracted to, and accumulated on, the surfaces of the dielectric films 12 and 14 on both sides, corresponding to the polarity of the pulse voltage. Since the equivalent internal voltage, namely, the wall voltage due to the accumulated charge has the polarity opposite to that of the pulse voltage, the effective voltage within the cell falls with the growth of the discharge. Accordingly, even if the pulse voltage is held at a constant level, it is unable to sustain the discharge, and eventually the discharge is ceased. After this happened, if a sustaining voltage which is a pulse voltage having the same polarity as that of the wall voltage is applied between adjacent scanning electrode 3 and the sustaining electrode 4, the wall voltage component is superposed as an effective voltage to it, so it is possible to sustain discharge beyond the discharge threshold even when the voltage amplitude of the sustaining pulse is small. Consequently, it is possible to sustain the discharge by the continuous application of the sustaining pulse between the scanning electrode 3 and the sustaining electrode 4. This is the memory function referred to in the above. The sustained discharge can be stopped by applying a broad low voltage pulse or a narrow erasing pulse, being a pulse comparable to the sustaining pulse voltage, capable of neutralizing the wall voltage between the scanning electrode 3 and the sustaining electrode 4.
Next, referring to a block diagram in FIG. 2 showing an example of the drive unit of the conventional PDP, the configuration of the PDP will be described. In the PDP, a sustaining electrode group 42 and a scanning electrode group 53 are provided mutually parallel on one surface, and a data electrode group 32 is provided in the direction perpendicular to these electrodes on the opposing surface. Display cells 22 are formed at the intersections of these arrays. The sustaining electrodes X are provided corresponding to respective scanning electrodes Y1, Y2, Y3, . . . , and Yn (n is an arbitrary positive integer) adjacent to them, and their respective one ends are connected in common.
Next, the configurations of plural kinds of driver circuit for driving the display cells 22 and a control circuit for controlling the driver circuits will be described. The drive unit is provided with a data driver 31 which drives one line portion of data of the data electrode group for the purpose of addressing discharge of the display cells 22, sustaining driver circuit 40 which performs common sustaining discharge for the purpose of sustaining discharge of the display cells 22, and a scanning driver circuit 50 which performs common sustaining discharge for the scanning electrode group 53. The sustaining driver circuit 40 and the scanning driver circuit 50 are composed of lowimpedance circuits and high impedance circuits as illustrated in FIG. 3. In addition, for the purpose of performing selective writing discharge during the addressing period, there is provided a scanning driver 55 which performs sequential scanning to the scanning electrodes Y1 to Yn of the scanning electrode group. The scanning driver 55 performs sustaining discharge by applying a sustaining pulse to its own power supply by means of the scanning driver circuit 50. A control circuit 61 controls all of the operations of the data driver 31, sustaining driver circuit 40, scanning driver circuit 50, scanning driver 55, and PDP 21. The main part of the control circuit 61 comprises a display data control part 62 and a drive timing control part 63. The display data control part 62 possesses a function of rearranging display data input from the outside to data for driving the PDP 21, stores temporarily the rearranged display data stream, and transfers them to the data driver 31 as display data DATA in synchronism with the sequential scanning of the scanning driver 55 during addressing discharge. The drive timing control part 63 converts various kinds of signal such as dot clock signal input from the outside into internal control signals for driving the PDP 21, and controls respective drivers and driver circuits.
Next, the drive sequence will be described. FIG. 4 is a diagram showing the state of formation of a plurality of sub-fields in the conventional drive unit of the PDP. In this example, a field having a period of 16.7 ms is divided into 8 sub-fields (abbreviated as SFs). It is arranged so as to be able to display 256 gradations by regulating the drive sequence through the combination of these sub-fields. Each sub-field is divided into a scanning period for writing display data corresponding to the weight of the sub-field, and a sustaining discharge period for displaying display data designated for writing. An image for one field is displayed by the superposition of respective sub-fields.
FIG. 5 is a diagram showing the details of a sub-field of prior art. The diagram shows a sustaining electrode driving waveform Wx that is applied in common to the sustaining electrodes, sustaining electrode driving waveforms Wy1 to Wyn applied to the sustaining electrodes Y1 to Yn, and data electrode driving waveforms Wdi (1xe2x96xa1ixe2x96xa1k) applied to the data electrodes D1 to Dk. One cycle of the sub-field is divided into a scanning period and a sustaining discharge period, where the scanning period is subdivided into a preliminary discharge period and a writing discharge period, and a desired image display is obtained by repeating this combination. The preliminary discharge period is employed as needed, and may be omitted.
The preliminary discharge period is the period for generating active particles and wall charges within the discharge gas space in order to prepare for obtaining a stabilized writing discharge during the writing discharge period. The period includes the application of a preliminary discharge pulse for simultaneous discharge of the entire display cells of the PDP, and a preliminary discharge erasing pulse for eliminating charges that hinder the writing discharge and sustaining discharge, among wall charges generated by the application of the preliminary discharge pulse.
The sustaining discharge period is the period for causing the display cell which was subjected to the writing discharge during the writing discharge period to undergo sustaining discharge for luminescence with desired luminance.
During the preliminary discharge period, first, a preliminary discharge pulse Pp is applied to the sustaining electrodes X to cause discharge in all display, cells. Then, an erasing discharge is generated by applying a preliminary discharge erasing pulse Ppe to the scanning electrodes Y1 to Yn to eliminate the wall charges accumulated by the preliminary discharge pulse.
Following that, during the writing discharge period, a scanning pulse Pw is applied line sequentially to the scanning electrodes Y1 to Yn, and a data pulse Pd is applied selectively to the data electrodes Di (1xe2x96xa1ixe2x96xa1k) corresponding to image display data, and wall charges are created by generating a writing discharge in a cell to be displayed. Then, during the sustaining discharge period, sustaining discharge is generated continuously only in the display cell which was subjected to writing discharge, by sustaining pulses Pc and Ps. After the final sustaining discharge is completed by a final sustaining pulse Pce, the formed wall charges are eliminated by a sustaining discharge erasing pulse Pse, and a luminescence operation for one image screen is completed by stopping the sustaining discharge. 
It is a first object of the present invention to provide a plasma display panel which enables to obtain a satisfactory image quality irrespective of the size of the display load amount.
First, in the conventional drive method of the plasma display panel, the times from the start of charge recovery to the clamping to the sustaining potential and to the ground potential are fixed to prescribed values. Accordingly, when the times from the start of charge recovery to the clamping to the sustaining potential and to the ground potential are set short, there is a disadvantage in that luminance saturation takes place especially when the display load amount is small, and satisfactory display image cannot be obtained due to an excessively strong gas discharge intensity. On the other hand, when the times from the start of charge recovery to the clamping to the sustaining potential and to the ground potential are set long, there occurs a disadvantage that a required luminance cannot be obtained when the display load amount is large due to low gas discharge intensity.
Moreover, in the conventional drive method of the PDP, a plurality of display cells are driven by one line formed by an electrode pair of a sustaining electrode X of the sustaining electrode group and a scanning electrode out of Y1 to Yn of the scanning electrode group. In this case, the display current corresponding to display data of each line is approximately proportional to the display data amount (load amount) in the display cell. Each electrode has a distributed resistance component, which is larger for larger electrode length. As a result, a voltage drop occurs when a display current is supplied by the resistance component of the electrode, where the amount of the voltage drop depends on the display data amount. Further, a stray capacitance exists between the electrodes to begin with, so a voltage drop occurs also due to unwanted charge accumulation caused by the stray capacitance.
Furthermore, the sustaining driver circuit 40 and the scanning driver circuit 50 of the conventional device are composed of the combination of low impedance circuits and high impedance circuits as shown in FIG. 3, or of low impedance circuits alone. Both of the total output and each control signal are common to both circuits, and the time at which a control signal for low impedance circuit is turned on is fixed as shown in FIG. 6 which is an enlarged diagram illustrating the trailing edge A of the sustaining pulse in FIG. 5. In this case, since the discharge current is always supplied by the low impedance circuit, a voltage drop is induced depending on the display data amount as in the above.
Because of this, the voltage drop remains small when the display data amount is small, but the voltage drop increases as the display data amount becomes large, causing a difference in the display luminance between the lines. Namely, as shown in the solid line of the graph showing the dependence of the luminance on the display load amount in FIG. 14, the luminance is unnecessarily high when the display data amount is small, whereas the luminance drops when the display data amount is large. As a result, there arise irregularities in gradations which should be gradual intrinsically, and causes a problem that the luminance characteristic is discontinuous.
The present invention was motivated in view of the above problems, and it is the object of the invention to provide a drive method and a drive unit of a plasma display panel with excellent display quality which is capable of faithfully displaying the gradations of the display data regardless of the size of the display load amount by suppressing the rise in the luminance when the display data amount is small, and by preventing the drop in the luminance when the display data amount is large.
In order to achieve the above object, the present invention adopts basically the following technical setup.
Namely, in a drive method of a plasma display panel comprising a plurality of display cells arrange in a matrix form, in which the luminescence after writing discharge is sustained by means of sustaining discharge pulses, this invention is characterized in that the sustaining discharge pulse has at least a plurality of timing patterns. By the use of the plurality of timing patterns the display load becomes adjustable, and a faithful display of the gradations of display data becomes attainable.
In addition, the drive unit according to this invention is a device for the plasma display panel which causes a sub-field to luminesce with prescribed gradations using n sustaining pulses, and is provided with a variable means which varies the time from the start of charge recovery of the sustaining pulse to the clamping to the sustaining potential and the time to the clamping to the ground potential. Moreover, the variable means is composed of a first switching means which clamps the sustaining pulse to the ground potential, a second switching means for clamping the sustaining pulse to the sustaining potential, a third switching means for guiding the charge on the display cell of the plasma display panel to a recovery capacitor, a fourth switching means for guiding the charge on the recovery capacitor to the display cell, and a control circuit for controlling the switching timings of the first to the fourth switching means.
Furthermore, the drive unit is provided with an arithmetic means for calculating the display load amount of the display cell, and the result of calculation of the arithmetic means is used to control the variable means. It is preferable that the time from the start of charge recovery of the sustaining pulse to the clamping to the sustaining potential, and the time to the clamping to the ground potential are increased successively from the leading sustaining pulse to the n-th sustaining pulse, or that the times are made variable corresponding to the display load amount.
It is preferable that the drive unit of the plasma display panel according to this invention has means for resetting each display cell, a writing discharge means for deciding lighting or nonlighting of each display cell, and a sustaining discharge means for performing repeated luminous discharge based on the selective discharge in the writing discharge means, and is provided with a means for detecting display data for performing writing discharge for each line, a means for counting and storing the display load amount of the detected display data, and a means for variably controlling dynamically the point of impedance change in the sustaining discharge means for each line, at switching of the sustaining pulse during the sustaining discharge, corresponding to the display load amount of the detected display data.
The sustaining discharge means is composed of high impedance circuits and low impedance circuits. The high impedance circuit is composed of a circuit generating a leading edge (trailing edge) of the sustaining pulse, and the low impedance circuit consists of a circuit for clamping the sustaining pulse to the sustaining voltage and a circuit for holding it at the sustaining voltage, and the circuit for generating a leading edge (trailing edge) of the sustaining pulse is configured in a form to include a reactive power recovery means.
The operations mentioned above to be performed for each line may be performed for each sub-field or for each field.